Dawn Porter’s engrossing HBO documentary profiles three public defenders in the Deep South. They work long hours for relatively low pay, representing people who are poor and sometimes contemptible: Brandy Alexander, one of the subjects, recalls a defendant proudly explaining to her how he went about raping his 12-year-old daughter; on another occasion, she says, a client unhappy with the outcome of his trial threatened her life in open court. Public defenders must take cases no other attorney would go near, and the reward is often sleepless nights wondering if they’ve helped a guilty person go free or failed to rescue an innocent one from prison. Senior public defender Brett Willis isn’t among the three subjects Porter follows, but he often provides the most knowing commentary. When people ask him how he can defend so many scumbags, he says, he asks them what their own freedom means to them. His job is “about the sanctity of human liberty, and the cost of it, if you want to take it.”